In U.S. Pat. No. 4,082,464 to Robert Lincoln Johnson, Jr., issued Apr. 4, 1978, there is disclosed an instrument for measuring and analyzing the optical properties of organic materials to determine the percentages of certain constituents of the test materials. This instrument was developed to satisfy a need for an instrument to rapidly determine the moisture, oil and protein content in grain products. In the instrument disclosed in the patent, a source of wide-band infrared light is positioned to illuminate a sample through a filter assembly in which interference filters are arranged on a wheel on a cylindrical locus. The wheel rotates to move the filters in sequence through a beam of light and to tilt each filter about an axis, thereby varying the angle of incidence of the light to the filters as the filters move through the beam of light, so that the wavelength passed by the filters is swept through a range of values. The intensity of light reflected from or transmitted through a test sample is detected to provide an indication of the reflective or transmissive optical density of the test samples at specific wavelengths. The angular position of each of the filters on the wheel is adjustable to make the range of light wavelengths transmitted by each filter adjustable. By detecting the amount of reflection at selective specific wavelengths and the relationships of these reflectivities, the oil, protein and water content of the sample can be accurately and quickly determined.
U.S. Ser. No. 45,089, filed June 4, 1979, now U.S. Pat. No. 4,400,086 discloses an instrument similar to the type disclosed in the Johnson patent, but in which the filters are in a paddle wheel arrangement and the material to be analyzed is ground automatically in the instrument at the time the measurement is made. In the invention disclosed in the application, a grinder is provided on the instrument with a hopper to hold grain and to introduce it into the grinder. A plate blocks the bottom of the hopper from the grinder so that the hopper may be filled with grain prior to a measuring operation. A reflectivity standard is positioned in the path of the infrared rays prior to each measurement to automatically calibrate the instrument. To initiate operation of the instrument, the reflectivity standard is pivoted out of the path of the infrared light. This action automatically energizes the grinder motor. Then, after a delay of several seconds to permit the grinder motor to get up to speed, grain is permitted to flow from the hopper into the grinder and the ground grain is permitted to flow into a chute, the bottom of which is arranged to receive the infrared light passing through the filter wheel. An impeller, a vibrating trough or other suitable device is mounted at the bottom of the chute to remove the grain from the chute. After another delay of several seconds, sufficient for the chute to fill up, the discharging device is actuated to begin to move the grain out of the bottom of the chute. At this time, the instrument begins to make measurements as the grain in the chute moves through the infrared beam. This provides an automatic averaging from the sample being analyzed.
In the aforementioned instruments, the sample has been illuminated with a light beam which has a large circular cross section, and a problem occurs because of the fact that, although the light is supposed to be columnated, the columnation of the light is not perfect and some light travels diametrically across the circular beam from one side to the other and this angularly directed light, when impinging upon the filters, will pass through the filters at a different angle from the angle of the light which is parallel to the axis of the beam. This results in a wavelength other than the desired wavelength being transmitted through the filter. This effect, called band width spreading, is greatest for the skewed light rays which are skewed from the axis of the beam in a plane perpendicular to the tilting axis of the filter, as compared with those rays which are skewed in a plane parallel to the tilting axis of the filter. This is due to the fact that the light rays which are skewed in the parallel plane will have less of an angular difference from the axial rays as the filter is tilted than will the skewed light in the perpendicular plane. Thus, as the filters are moved through the beam and are tilted relative thereto, the light rays which are skewed in the plane parallel to the tilt axis will introduce a smaller error into the measurement than will the skewed light rays in the plane perpendicular to the tilt axis.
Accordingly, in order to eliminate the light rays which introduce the greatest error into the measurements, the light beam, which has a circular cross section, is passed through a rectangular aperture, so that those skewed light rays which fall farthest from the plane parallel to the tilting axis of the filter are prevented from reaching the sample. Thus, the light beam which does reach the sample has a cross section in the shape of a long narrow rectangle. As a result, the band width spreading due to the skewed light in the plane perpendicular to the tilting axis is greatly reduced. No improvement in the spreading in the plane parallel to the tilting axis is achieved, but, as explained above, this band width spreading occurs to a lesser extent than in the perpendicular plane and causes less of an error.
As the individual filters are swept through the light beam, measurements can only be made when the entire light beam is shining through a particular filter. Since the dimension of the light beam lying in a plane perpendicular to the tilting axis of the filters is greatly reduced by the present invention and since the reduced dimension is parallel to the direction of the movement of the filters, the entire light beam impinges on a given filter for a greater amount of travel of the filter and, therefore, through a greater angle of tilt of the filter as the filter is swept through the beam. Thus, a greater scan of the wavelength with each filter is achieved.
A disadvantage of the rectangular beam is that it illuminates a smaller area of the sample. However, this problem is overcome by using the technique of the ultraspeed detector as disclosed in Ser. No. 45,089, U.S. Pat. No. 4,400,086, in which the grain sample is moved continuously past a window as the sample is measured in order to expose a large area of the sample to the beam. Alternatively, a sample container can be transported past the window in order to get averaging over a sufficient area of the sample to get an accurate measurement.